Study initiated to determine if erythrocyte lesions are present in those rare cases of magnesium deficiency in man with concomitant low red cell magnesium, normal controls, cases of Bartter's syndrome with isolated magnesium deficiency and cases of idiopathic magnesium deficiency. All cases are examined before and after Stractan separation. Initial results indicate that magnesium deficiency in man produces both a membranous and an intracorpuscular defect.